The amount of video image data is large in general. Especially, the amounts of video image data, which conforms to standards that are 4K and 8K ultra-high-definition standards and the like and are related to a very large number of pixels, are significantly large in some cases. Thus, before a certain device that handles video image data transmits video image data to another device or to cause video image data to be stored in a storage device, the certain device compresses and encodes the video image data. As representative video image coding standards, there exist Moving Picture Experts Group Phase 2 (MPEG-2) developed by International Standardization Organization/International Electrotechnical Commission (ISO/IEC), MPEG-4, and H.264 MPEG-4 Advanced Video Coding (MPEG-4 AVC/H.264). In addition, as a new standard, High Efficiency Video Coding (HEVC, MPEG-H/H.265) has been developed.
On the other hand, a certain decoding device may not have computing power sufficient to decode encoded video image data and almost completely maintain the original quality of the video image data. Thus, as a technique for adaptively distributing video image data with different qualities based on the computing power of the decoding device, a hierarchical coding technique (also referred to as scalable coding) has been proposed. In the scalable coding, upon the decoding of encoded video image data, the resolution or frame rate of pictures included in the video image data may be reduced from the original resolution or frame rate. Due to the reduction in the resolution or frame rate, the decoding may be executed with low computing power.
In addition, it may be acceptable that while partial regions of pictures are reproduced with the original qualities, other regions of the pictures are reproduced with low qualities, depending on the use of video image data. Thus, a technique for switching between a block to be encoded in YCbCr 4:4:4 format and a block encoded in YCbCr 4:2:0 format on a block basis for each picture has been proposed. YCbCr 4:4:4 format is one of formats for representing pictures. In YCbCr 4:4:4 format, a value of each pixel is represented by a luminance signal Y and two color-difference signals Cb and Cr, and the spatial resolution (or picture size) of the luminance signal Y is equal to the spatial resolution of the two color-difference signals Cb and Cr. In YCbCr 4:2:0 format, two color-difference components (Cb and Cr) are downsampled to ½ in vertical and horizontal directions, compared with YCbCr 4:4:4 format.
In the conventional technique, a pixel value is checked for each of blocks within a picture, and the encoding using YCbCr 4:4:4 format and the encoding using YCbCr 4:2:0 format are switched based on the results of the checking. For example, a block including a small linear structure such as a character region is encoded in YCbCr 4:4:4 format, and a block including a video image generated by a camera is encoded in YCbCr 4:2:0 format. In this case, since the number of blocks encoded in YCbCr 4:4:4 format is minimal, the amount of video image data to be encoded is suppressed.
Examples of the related art include International Publication Pamphlet WO2015/182342 and Non-Patent Document (T. Lin, S. Wang, “Mixed Chroma Sampling-rate coding: combining the merits of 4:4:4 and 4:2:0 and increasing the value of past 4:2:0 investment”, JCTVC-H0065, 8th JCT-VC Meeting, San Jose, USA, 1-10 Feb., 2012).